The invention relates to an adaptive vehicle drive system which shifts from two-wheel drive to four-wheel drive upon sensing certain conditions, and more specifically to an adaptive system which detects high magnitude and rapid repetition wheel spin transients and shifts to four wheel drive operation for a predetermined time period.
The performance advantages of four-wheel vehicle drive systems are well recognized. Improved vehicle stability while traversing rain soaked or ice or snow covered highways, handling and control on gravel or uneven pavement and simply maintaining traction in off road situations are all readily acknowledged benefits. Concomitant and less desirable attributes of four-wheel drive systems relate to reduced gas mileage from increased drive line friction and increased vehicle weight. Such increased drive line friction occurs in part time four-wheel drive systems which rotationally couple the front and rear vehicle propshafts. Such vehicle weight increases are particularly pronounced if the system is designed with a differential between the front and rear drive shafts for full-time engagement and operation rather than intermittent operation when conditions specifically demand it.
Furthermore, while part time four-wheel drive systems which lock the front and rear propshafts together provide obvious benefits of traction and stability in straight line driving, the disparity between the ground speed at the front wheels and the ground speed at the rear wheels during cornering can itself result in wheel slip and hopping of the vehicle. Thus, allowing the front and rear output shafts of the transfer case to operate at different speeds during cornering is beneficial.
Many four-wheel drive systems employing diverse control and torque distribution strategies have been designed and utilized. These various approaches are embodied in United States patents.
For example, U.S. Pat. No. 4,417,641 teaches a four-wheel drive system having an electromagnetic clutch and steering sensor. When the steering wheels are turned greater than a predetermined angle, the electromagnetic clutch is de-energized, disconnecting two drive wheels.
U.S. Pat. No. 4,718,303 discloses a transfer case having an electromagnetic ramp clutch which is modulated to adjust torque distribution in a full time four-wheel drive system. In U.S. Pat. No. 4,937,750, a microcomputer compares signals from front and rear driveline speed sensors. If the difference is greater than a certain value, a clutch is engaged to interconnect the front and rear drivelines.
U.S. Pat. No. 4,989,686 discloses a four-wheel drive system including wheel speed detectors. The detectors control a proportional clutch which delivers torque to whichever driveline is rotating more slowly. U.S. Pat. No. 5,002,147 discloses a four-wheel drive system which achieves torque splitting between the front and rear axles. The system utilizes four wheel speed sensors and a steering angle sensor.
In U.S. Pat. No. 5,060,747, a torque distribution system is taught which includes both vehicle and front and rear wheel speed sensors. Vehicle speed data is utilized to adjust the wheel speed difference value and this adjusted value is utilized to engage a clutch.
U.S. Pat. No. 5,090,510 discloses a four-wheel drive system having a differential and a hydraulic clutch disposed in parallel between the front and rear drive shafts.
A nearly universal problem of the foregoing active torque distribution systems is their operation in extreme off-road conditions, such as sand or mud, where randomly repeated, high magnitude speed difference transients repeatedly activate and deactivate the torque distribution clutch. Such operation is often justly characterized as unpleasant by the vehicle operator and occupants because of the abrupt, random and repeated cycling of the torque distribution clutch which is counter to the smooth, adaptive torque distribution goal of such systems. A control strategy that will recognize operation under such conditions and provide a smooth and comfortable operational solution to such conditions is therefore desirable.